The original Transport Tycoon and Transport Tycoon Deluxe games are unfortunately quite hard to find these days.
The Where To Get It page offers some suggestions for where to find the
game to purchase.

Conveniently though, the OpenTTD project has now been able to recreate all of the Transport Tycoon Deluxe graphics,
sound and music, so the game no longer relies on any of the original content (although you can use it if you happen to have
a copy of the original game).